Method of thermal treatment of a wafer in an evacuated environment

ABSTRACT

In a vacuum chamber wafer treating apparatus a wafer is heated or cooled by introducing a gas at a pressure of approximately 100 to 1000 microns in a region between the wafer and a heating element or heat sink. The gas conducts thermal energy between the wafer and heating element or heat sink.

The present application is a continuation of Ser. No. 366,433 filed Apr.7, 1982, now U.S. Pat. No. 4,680,061, issued July 14, 1987, in turn adivision of Ser. No. 106,343 filed Dec. 21, 1979.

BACKGROUND OF THE INVENTION

The present invention relates to the coating of thin substrates bydeposition under vacuum. More particularly, the field of the inventionis metallization of semiconductor wafers, and apparatus for effectingsuch metallization of wafers individually, and on a serial, continuousbasis. Semiconductor wafer fabrication techniques have evolved rapidlyover the past decade. Individual microcircuit devices have becomeprogressively smaller, thereby increasing the number of such devicesthat can be put onto a wafer of a given size. Additionally, wafers oflarger diameter are coming into use. A few years ago wafers of 2-inchdiameter were commonplace, and 3-inch diameter wafers were consideredlarge. Today much of the device fabrication is done with 4-inch diameterwafers and widespread use of 5-inch wafers within a very few years isforeseen. The reductions in device size, coupled with the increased sizeof wafers, have served to greatly increase the economic value ofindividual wafers, and thus the need to process and metallize suchwafers in an improved manner.

Most semiconductor and microcircuit fabrication techniques requiredeposition of metallic coatings of high quality upon the semiconductorwafer upon which the microcircuits are defined. Whether a coating is of"high" quality will of course ultimately be determined by the degree ofsatisfaction with the ultimate yield of microcircuit devices from thewafer, and their utility, for example, as meeting the higher military orindustrial standards, or the lesser consumer and hobbyist standards.Although therefore difficult to quantify, it is generally agreed thatmetallization quality, and thus ultimate quality and quantity of yield,will in turn be a function of the following factors: uniformity ofcoverage upon the uppermost and main planar surface of the wafer("planar coverage"); contamination levels incorporated into the finalcoating; defect level caused by debris; symmetry and homogeneity, or inother words freedom from "layering" and the manner of distribution ofcontaminant levels in the film; the degree of reproducibility andcontrol, especially of temperatures during coating deposition; and stepcoverage, that is, the continuity and evenness of the coating across notonly the main plane of the surface, but also the sides and bottoms ofsuch features within the surface as steps, grooves, depressions, andraised portions which define the microcircuits.

Some of these characteristics are harder to achieve or are more criticalthan others, or have been thought to require definite specializedprocessing steps to achieve. For example, because of the constraints ofgeometry, step coverage has been a particularly difficult requirement tofulfill. The sidewalls of the steps and grooves are generallyperpendicular to the uppermost surface of the main plane of the wafer,and may face both inwardly and outwardly of the wafer center. Coveringof such perpendicular surfaces, particularly the outer-facing ones,while at the same time covering the planar surfaces, is obviously anespecially difficult problem, yet such "step coverage" is of particularimportance in determining the quality of the metallization overall. Ithas generally been thought that in order to achieve the requireduniformity of planar surface coverage as well as adequate step coverage,relative motion between the wafers and the deposition source duringcoating deposition is necessary. However, such motion carries with itcertain disadvantages, especially the heightened possibility ofgeneration of debris, as by dislodgement of deposits of coating materialon various internal structures of the apparatus due to the motion, aheightened possibility of mechanical shock and vibration damage to thewafer, and the buildup of deposition on the wafers in a nonsymmetricaland inhomogeneous fashion, as will be further explained below.Naturally, contamination level will depend on the maintenance of thequality of the vacuum environment during deposition and theconcentration of contaminants relative to the speed of deposition. Thusthe adequacy of "outgassing", or the evacuation of gas and vapors fromthe wafer and accompanying wafer supports which are introduced into thecoating chamber will also be important.

The manner in which the prior art has attempted to achieve one or moreof the above characteristics, and the attendant difficulties andtrade-offs involved in achieving the above criteria of coating quality,may be best appreciated by considering the two main types of vacuumdeposition systems which are in current use for metallizing wafers:batch and load lock. A typical batch system comprises a pumping station,an evacuable bell jar, an isolation valve between the pumping stationand the bell jar, heat lamps, one or more deposition sources, andplanetary fixtures which hold the semiconductor wafers and rotate themabove the deposition source or sources. At the start of a depositioncycle the isolation valve is closed and the bell jar is open. Wafers areloaded manually from cassettes into the planetary fixtures (a load of 75wafers of 3-inch diameter is typical). The planetary fixtures are thenmounted in the bell jar, the bell jar closed, and the system evacuated.After a prescribed base pressure is reached, the wafers are furtheroutgassed through the application of radiant energy from the heat lamps.In some cases the wafers are sputter-etch cleaned prior to the start ofdeposition. A typical coating is aluminum or an aluminum alloy sputteredonto the wafer to provide interconnect metallization. In order toachieve the required coating uniformity and step coverage, relativemotion is provided by rotation of the planetary fixtures. Afterdeposition, the wafer and system are allowed to cool, the isolationvalve is closed, the bell jar vented to atmosphere, the bell jar opened,and the planetary fixtures are removed and unloaded manually intocassettes. This completes a typical cycle, which takes approximately 1hour.

Although such batch systems are in widespread use in production todayfor metallizing semiconductor wafers, certain of their characteristicspose limitations and disadvantages. For one, the entire relatively largebatch of wafers is inherently "at risk" of a partial or total lossduring the deposition cycle. The manual loading of wafers from cassettesinto planetary fixtures provides ample opportunity for contamination andbreakage. Air exposure of the entire system inside the bell jar forloading and unloading leads to possible contamination and adds a verylarge outgassing load for the vacuum pumps to contend with (theoutgassing area ascribable to the wafers alone is typically less thanten percent (10%) of the total air-exposed area which must beoutgassed). Long deposition throw distances (typically 6 to 14 inches)from the source are needed to obtain the large area coverage for themany wafers to be coated within the batch system. This leads to lowdeposition rates (typically 600 angstroms per minute for sputterdeposition source), which make the films more susceptible to poisoningby reaction with background gases, and thus more sensitive to thequality of the evacuated environment. Outgassing of the wafers and theair-exposed areas of the system is accelerated by application of radiantenergy from heat lamps, but since the wafers are in uncertain thermalcontact with the planetary fixtures, their temperatures are alsouncertain. Moreover, the heating source normally cannot be operatedduring sputter deposition, so that the wafers cool in an uncontrolledfashion from the temperature attained during preheat. Lack of control ofwafer temperature during deposition limits certain aspects of filmcharacteristics which can be reliably and reproducibly attained. Ofcourse the mechanical motion of the planetary fixtures for achievinguniformity and step coverage can dislodge particles of coating materialdeposited elsewhere within the system other than on the wafers, which inturn can cause debris to become attached to the wafers, in turn reducingyield of good devices.

A typical load lock system comprises a pumping station, an evacuableprocessing chamber, an isolation valve between the pumping station andthe processing chamber, a heating station, a deposition source, a loadlock, and a platen transport system. At the start of a deposition cycle,wafers are loaded manually from a cassette into a metal platen (a12-inch by 12-inch platen size is typical), which then acts as a carrierfor the wafers during their journey through the load lock and processingchamber. After introduction through the load lock into the processingchamber, the platens and wafers are transported to the heating station,where they are further outgassed by the application of radiant energy.Additional cleaning of the wafers by means of sputter etch may also beperformed at the heating station. Metal film deposition is accomplishedby translating the platen and wafers relatively slowly past thedeposition source, which may be a planar magnetron type of sputteringsource with a rectangular erosion pattern, with the long dimension ofthe erosion pattern being greater than the platen width. Relatively highdeposition rates (10,000 angstroms per minute) are achieved by movingthe platen past the sputter source over a path which passes the waferswithin several inches of the sputter source. After deposition, theplaten and wafer are returned to the load lock where they pass from theprocessing chamber back to atmosphere. The wafers are then unloadedmanually back into a cassette. This completes a typical cycle, whichtakes typically 10 to 15 minutes. In another type of load lock system,the wafers are mounted on an annular plate which rotates past thedeposition source. Each wafer makes multiple passes below the depositionsource until a film of sufficient thickness is built up.

The above load lock systems overcome some of the disadvantages of batchsystems, but not all. Of primary importance is the fact that the use ofa load lock allows wafers on a platen to be introduced into and removedfrom the processing chamber without allowing the processing chamberpressure to rise to atmospheric. This greatly reduces the amount ofair-exposed surface that must be outgassed prior to deposition. Whilethe processing chamber does need to be opened to atmosphere periodically(for cleaning and for replacing deposition targets), the frequency ofsuch air exposure is much less than with batch systems.

Another important factor is that the size of the wafer load which is "atrisk", that is, subject to being rejected due to a defect or failure ofthe process, is significantly smaller in the load lock system (16 3-inchwafers in the first load lock system, compared with 75 3-inch wafers inthe batch system in the above example). Because the number of wafers perload is much smaller with the load lock system, it is not necessary toemploy the long deposition throw distances required with batch systems.Higher deposition rates are therefore attainable by closer couplingbetween wafer and source.

Despite the advantages afforded by load lock systems, many disadvantagesand shortcomings still remain. In both batch and load lock systems,wafers are typically transferred manually between platen and cassette,with attendant risks of contamination and breakage. Although the use ofthe load lock avoids exposure of the processing chamber to theatmosphere, the platen which carries the wafers is air-exposed on eachload and unload cycle. Thus its surfaces must also be outgassed, whichraises the total outgassing load well beyond that of only the wafersthemselves. In addition, sputtered deposits that build up on the platenbecome stressed due to repeated mechanical shock and air exposure,leading to flaking and debris generation. As with batch systems, wafersare still in uncertain thermal contact with their carrier. Controls overwafer temperature during outgassing and during deposition remaininadequate. The metal films are put down on the wafers in anonsymmetrical fashion, since the film deposited on the wafer builds upin different ways depending upon its location on the platen, i.e.,whether the wafer is outboard, inboard, approaching the source, ormoving away from the source. Translation of the platen during depositionto provide uniformity and step coverage heightens the risk of generationof debris and flakes, and thus contamination of the wafers. In certainload lock systems, symmetry and homogeneity are further compromised bycausing the wafer to make multiple passes below the deposition source.Thus the metal film is deposited in a "layered" fashion because thedeposition rate tapers off to almost nothing when the wafers arerotating in a region remote from the deposition source. The low rates ofdeposition in such regions heighten the risk of contamination due toincorporation of background gases into growing film, andnon-uniformities in the distribution of any contaminants which may bepresent result from the non-uniformities in deposition rate.

Even though a much smaller number of wafers is being processed at anyone time in load lock systems as compared with batch systems, asignificant number of wafers still remains "at risk". From this point ofview, it would be best to process wafers individually on a serialcontinuous basis, but the time needed for adequate pumpdown of the loadlock during loading and unloading, and for wafer outgassing and theoutgassing of wafer supports, coupled with the time needed to coat awafer individually in an adequate manner, has rendered the concept ofsuch individual processing impractical until now as compared to batchsystems or load lock systems handling a plurality of wafers with eachload. It would also be much better from the viewpoint of prevention ofdebris generation and consequent reduction in yield of good microcircuitdevices, as well as lessening the risk of abrasion and mechanical shockand vibration, to hold wafers stationary during coating deposition.However, as we have seen, this has been considered inconsistent with theneed to obtain adequate deposition uniformity and step coverage, sincethis normally requires establishing relative motion between the sourceand wafer. Further, there has been no basis for expecting greatercontrol over reproducibility and temperature of the coating process in aindividual wafer processing system as opposed to a batch or load locksystem coating a plurality of wafers with each load.

Accordingly, an object of the invention is to provide apparatus forrapidly coating wafers individually with a higher quality coating thanpossible previously.

A related object of the invention is to provide apparatus for depositingmetallization layers of superior quality with respect to the aggregateconsiderations of step coverage, uniformity, symmetry and homogeneity,contamination level, debris damage, and reproducibility.

Also an object of the invention is to provide apparatus for rapidlycoating wafers individually with improved step coverage and gooduniformity.

Another related object of the invention is to provide an improved loadlock system for metallizing wafers individually yet at a high rate.

Yet another object of the invention is to provide an improved load locksystem for metallizing semiconductor wafers individually on aproduction-line basis with enhanced quality, including uniformity andstep coverage.

A related object is to provide a system for coating wafers which reducesthe number of wafers at risk at any one time due to processing.

Another related object is to provide a system for metallization or othervacuum processing of wafers individually on a serial continuous basis,with a plurality of work stations operating simultaneously on individualwafers.

Also a related object is to reduce the outgassing load and minimizedisturbance to the evacuated coating environment due to introduction ofwafers into a load lock system for coating.

Yet another object of the invention is to improve the yield ofmicrocircuit devices subsequently derived from the wafer by reducinggeneration of debris and the probability of damage from abrasion andincorporation of contaminants.

Yet another object is to provide a load lock type system whichaccomplishes transport between various work stations and into and fromthe vacuum regions without the use of platen-like supports for thewafers.

Also a related object of the invention is to provide a load lock systemas above which does not use platen-like wafer supports, in which loadingand unloading are effected to certain wafers while yet others are beingprocessed.

A further related object is to provide a system as above which iscompatible with automatic wafer handling from cassettes.

Also a related object is to provide improved control over wafers,especially their temperature, throughout the processing thereof.

Yet a further object of the invention is to provide a system forproduction-line use in which reliability, maintainability, and ease ofuse are improved.

SUMMARY OF THE INVENTION

The broadest objects of the invention are met by providing apparatus forcoating a wafer individually, which includes a ring-shaped sputteringsource emitting coating material and having a diameter larger than thatof one of the wafers, means for locating an individual one of the wafersin facing stationary relationship to the source, and at a distance lessthan the diameter of the source, as well as means for maintaining thesource and wafer in an argon environment of up to 20 microns pressure (1micron=10⁻³ millimeters of mercury=1 millitorr=0.133 Pa) during coatingof the wafer. In this manner a coating of improved quality with gooduniformity is rapidly deposited on the wafer without need for relativemotion between wafer and source and the added complexity and debrisgeneration risk attendant thereto.

The objects of the invention are also met by providing apparatus forrepetitively sputter coating individual wafers in minimal time, usefulwith vacuum chamber means continuously maintaining a controlledsubatmospheric environment. The apparatus includes internal wafersupport means positioned within the chamber immediately inside of theentrance thereof, including means for releasably and resilientlygripping edgewise an individual wafer, to immediately accept a waferupon insertion and permit instant release and removal upon completion ofcoating. Also included is a sputtering source mounted in the chamber andhaving a cathode of circular outline emitting coating material upon thewafer in a pattern approximating that from a distributed ring source.The source has a diameter which is larger than that of the wafer and isspaced from the wafer by a distance less than that of the sourcediameter. The wafer support means holds the wafer stationary duringcoating. Finally, the apparatus includes load lock means including amovable member within the chamber to seal the wafer support means offfrom the remainder of the chamber interior and when the door of theentrance is opened, isolating the wafer and support means from thechamber environment during insertion and removal of a wafer. In thismanner, individual coating of wafers can be repetitively performed, withdisturbances to the chamber environment by outside wafer supportexpedients, consequent contaminants, and large load lock volumesminimized, while overall wafer coating time is improved.

Objects of the invention are also met by apparatus for individuallyprocessing wafers continuously in a controlled subatmosphericenvironment, which includes a vacuum chamber having a first opening in afirst wall thereof and a door for closure of the opening, at least onewafer processing means mounted in a wall of the chamber and defining atleast one processing location of the chamber spaced from the firstopening, and movable carrier means within the chamber movable betweenthe first opening and the processing location. The carrier means isprovided with at least two apertures and spaced a first distance toenable the apertures to be aligned respectively with the first openingand the processing location. Each of the carrier apertures mounts clipmeans for releasably and resiliently gripping a wafer. The apparatusalso includes closure means within the chamber for closing off one ofthe carrier apertures when said aperture aligns with the first chamberopening, the closure means and chamber defined therebetween a small loadlock volume with the closure means sealing off the aperture from thechamber when the chamber door is opened to load or unload a wafer intothe clip means. In this manner wafers can be continually seriallyintroduced into the vacuum chamber with minimal disturbance of thecontrolled atmosphere therein, and the wafers individually processed atthe processing location while loading and unloading of another wafertakes place at the load lock without use of external wafer supportmeans, the presence of which would greatly increase the gas load to beeliminated by the load lock and chamber, as well as increasing thepossibility of contaminants. Furthermore, the load lock volume isminimized to only that absolutely necessary to accommodate a singlewafer, thus also reducing the amount of pumpdown load for the load lockand chamber.

In one preferred embodiment, the movable carrier means may be providedin the form of a disc-like transfer plate mounted for rotation about itsaxis with various wafer processing stations symetrically disposed aroundthe same axis. Besides sputtering stations, the stations may also beheating or cooling stations, and the heating, for example, may beapplied to the side of the wafer opposite that of the sputteringdeposition, since the clip means support the wafers edgewise, therebyallowing processing of both sides thereof. The wafer transfer platepreferably rotates in a vertical plane to better combat accumulation ofdebris on the wafers. When fully loaded, the apparatus limits the numberof wafers at risk at any one time to only those loaded in the wafertransfer plate, and enables several processing operations to be carriedon at the same time, for example, coating of one wafer simultaneouslywith the heating of another and with the unloading and loading of stillothers. The use of the internal wafer clip support means, thin loadlock, and individual processing of wafers enables easy loading andunloading, including simplified automatic loading. In one particularaspect, a vertically-acting blade-like elevator means raises a waferedgewise to a point immediately adjacent the chamber entrance. Vacuummeans associated with the door of the chamber then grip the rear surfaceof the wafer and propel same into the clip means as the door is closed,thereby loading the load lock and sealing same simultaneously. Furtherdetails of a fully automatic system for loading the vacuum processingchamber from a conveyor-driven cassette containing a plurality of wafersto be coated may be found in the co-pending application of G. L. Coad,R. H. Shaw, and M. A. Hutchinson for "Wafer Transfer System", filedcontemporaneously herewith, now U.S. Pat. No. 4,311,427. Similarly, thedetails of the means for resiliently supporting the wafers within thevacuum chamber and associated means for aiding the loading and unloadingof same into said supports within a chamber may be found in theco-pending application of R. H. Shaw for "Wafer Support Assembly", filedcontemporanously herewith, now U.S. Pat. No. 4,306,731.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of the complete wafer coating system ofthe invention, showing the main cylindrical processing chamber, the doorarrangement at the entrance of the load lock to the chamber, and thefour remaining work stations of the processing chamber, together withportions of the wafer cassette load/unload assembly;

FIG. 2 is a broken-away perspective view of the processing chamber ofFIG. 1, showing the load lock and sputter coating stations in moredetail;

FIG. 3 is a perspective view of the cassette load/unload assembly ofFIG. 1, showing its manner of cooperation with cassettes ofvertically-oriented wafers and the door assembly of the processingchamber, and the manner in which wafers are transferred therebetween andinto the chamber load lock;

FIG. 4 is a cross-sectional elevational view of the door and load lockof FIGS. 1 through 3, showing the manner in which the door assemblyloads a wafer into the load lock, and the manner in which the load lockis sealed from the remainder of the processing chamber interior;

FIG. 5 is a view similar to FIG. 4, showing the relative positions ofthe elements of the load lock upon completion of loading of the wafertherewithin;

FIG. 6 is a view similar to FIGS. 4 and 5, showing the position of thewafer and the load lock elements just subsequent to the extraction of awafer from the internal wafer support assembly, and prior to the openingof the door, or just before the loading of a wafer into the internalwafer support assembly immediately after closure of the door forloading;

FIG. 7 is a cross-sectional elevational view taken across line 7--7 ofFIG. 1, showing a wafer heating station within the chamber of FIG. 1;

FIG. 8 is an elevational cross-sectional view taken along line 8--8 ofFIG. 1, showing a wafer cooling station within the wafer processingchamber of FIG. 1;

FIG. 9 is a cross-sectional elevational view taken along line 9--9 ofFIG. 1, showing the wafer sputtering station of the chamber of FIGS. 1and 2;

FIG. 10 is a schematic cross-sectional view focusing on the wafer andsputtering source target of FIG. 9, which shows the spatialrelationships, relative positioning, and dimensions of these elements ingreater detail;

FIG. 11 is a graph illustrating the uniformity of thickness ofdeposition on the main planar surface of the wafer by the source ofFIGS. 9 and 10 as a function of the radial position on the wafer, foreach of several wafer-to-source distances;

FIG. 12 is a plot similar to FIG. 11, but for only a singlewafer-to-source distance, one curve being for a deposition environmentof 2 microns argon pressure and the other being for a depositionenvironment of 10 microns argon pressure;

FIG. 13 plots thickness of coating coverage on the sidewall of grooveswithin the wafer surface as a function of the radial position on thewafer, both for outwardly-facing (relative to the center of the wafer)sidewalls, as well as inwardly facing sidewalls, for a source-to-waferdistance of 4 inches, one curve being taken at 10 microns argonpressure, and the other at 3 microns argon pressure;

FIG. 14 is a plot similar to FIG. 13, except that the source-to-waferdistance is 3 inches;

FIG. 15 is a plot of uniformity of planar coverage as a function ofargon pressure of the coating environment, one curve being for theradial position 1.5 inches, and the other being for the radial position2.0 inches.

DETAILED DESCRIPTION

The wafer coating system of FIG. 1 principally includes a generallycylindrical vacuum processing chamber 10 having five work stations, oneof which comprises load lock arrangement 12; and one of which comprisescoating station 14. Remaining further elements of the coating systemfound within chamber 10 may be seen in more detail in FIG. 2, which alsoshows a wafer 15 within load lock 12; and also a wafer at coatingstation 14. Further elements include pressure plate 16, wafer carrierplate assembly 18, and clip assemblies 20 (better shown in FIG. 3), bywhich a wafer is retained within wafer carrier plate assembly 18. Doorassembly 22, which seals the entrance opening 23 of chamber 10, andwhich cooperates with the just mentioned elements to form the chamberload lock arrangement 12, completes the principal elements of processingchamber 10. These elements, together with cassette load/unload assembly24 and the various ancillary vacuum pumps 25 for chamber and load lockevacuation, and control means, are all housed compactly in cabinet 26.

The system desirably includes several other work stations other thanload lock arrangement 12 and coating station 14, in particular waferheating station 28, auxiliary station 29, and wafer cooling station 130.All five work stations are equally spaced laterally from each other andfrom the central axis 36 of the vacuum chamber. Although five stationsare provided, the design could be equally well adaped to both a greateror fewer number of stations. Also included are at least two pneumaticrams 30 and 31 which function to drive pressure plate 16 and wafercarrier plate assembly 18 against the front wall 32 of chamber 10, andcarrier plate drive 35, which centrally mounts carrier plate assembly18, which is circular and of nearly the diameter of front wall 32, forrotation about the central axis 36 of the vacuum processing chamber.

In general, wafers are individually presented and loaded by doorassembly 22 into load lock arrangement 12 and thereby within wafercarrier plate 18. The wafer is then passed in turn to each of the workstations, where it is heated for completion of outgassing and/orsputter-etch cleaned, coated, optionally coated layer, cooled, and thenreturned again to load lock 12 for removal from wafer carrier plateassembly 18, again by door assembly 22. Although the foregoinggenerally-described system is a rotary one and a multi-station one, theload lock and coating steps are equally applicable to a single stationor dual station configuration, or a non-rotary or in-line arrangement aswell.

Now considering the system in more detail from the view point of anincoming wafer, the load lock arrangement 12 through which a wafer 15must be passed in order to enter the evacuated environment of thechamber is of key importance. FIGS. 4-6 are especially important inappreciating the operation of the movable elements of load lock 12. Aspointed out above, the load lock is defined by a sandwich arrangement ofelements between the chamber door assembly in its closed positionagainst the front wall of the processing chamber and the pressure platein its driven position. The load lock is built around a circularaperture 37 within wafer carrier plate assembly 18, which is positionedinternally of the chamber just inside the chamber entrance 23 associatedwith load lock 12, with plate assembly 18 generally parallel to wall 32and the pressure plate 16, positioned within the chamber rearwardly ofplate assembly 18. Wafer 15 is loaded and held within the load lock andwithin the plate assembly by means which will be described below. Thecontrolled subatmospheric environment which may be provided withinchamber 10 for certain wafer processing operations may be, for example,up to 20 microns of argon or other inert gas for sputter coatingoperations. Because of this evacuated environment, the load lock regionmust be sealed off from the remainder of the chamber interior wheneverdoor 22 is open in order to preserve the evacuated environment. Pressureplate 16 serves the function of isolating the load lock area from thechamber interior (and also several other functions simultaneously atother work stations, as will be shown below). Pneumatic rams 30 and 31,mounted to the rear plate of the processing chamber, drive the pressureplate and plate assembly against front chamber wall 32, with pneumaticram 30 being applied particularly to the pressure plate concentricallywith load lock arrangement 12 to effect the sealing of the load lock.Both pressure plate 16 and chamber front wall 32 are equipped withO-rings 38 arranged in a circular pattern concentric with chamberentrance 23 provide vacuum seals in the sandwich arrangement of elementsdefining the load lock. Chamber door assembly 22, which in its closedposition seals against the outside surface of chamber front wall 32 andalso includes a concentric O-ring 39 to provide the vacuum seal,completes the load lock by sealing off the chamber entrance 23 from theoutside atmosphere. FIGS. 4 and 5 show the completed load lock, withpressure plate 16 in its forward, advanced position, compressing plateassembly 18 against chamber wall 32, and sealing off aperture 37; anddoor 22 closed to seal off chamber entrance 23 to form the load lockabout aperture 37, which is only of a size no larger than necessary toaccommodate a single wafer. It may be seen that an unusually thinlow-volume load lock is thereby defined with a minimum of elements, andof a minimum size necessary to accommodate wafer 15 therewithin. Forfurther details of the load lock arrangement, see the above-mentionedcopending aplication "Wafer Transport System". FIG. 5 shows pressureplate 16 in its withdrawn, rest position, and with the wafer alreadysecured within plate assembly 18 within the chamber.

Cooperating with this thin load lock arrangement is wafer carrier plateassembly 18, which includes a plurality of circular apertures such as at37 (as best seen in FIG. 2) matching the number and spacing of workstations within chamber 10. The apertures 37 are of a diameter largerthan the wafers, are equally spaced from each other, and centered at thesame radial distance from the central axis of the processing chamber.The aforementioned work stations are likewise spaced, so that when anyaperture of the wafer carrier plate assembly 18 is aligned with any workstation of the processing chamber, the remaining apertures are eachsimilarly aligned with a respective one of the remaining work stations.Thus, if a wafer is secured within each of the apertures of carrierplate 18, each of such wafers can be individually processed at a workstation simultaneously with the processing of other wafers respectivelyat the remaining work stations. In this manner, a wafer is individuallyprocessed at a given station, yet during the same time, several otherwafers may also undergo other operations at the remaining work stations.In particular, while a wafer is being unloaded and/or loaded at loadlock 12, another wafer may be coated at coating station 14, while stillanother wafer may be heated at heating station 28. Carrier plate drive35 intermittently operates to move plate assembly 18 by the distance ofone station so as to serially present each of the wafers in turn to eachof the processing stations in a counterclockwise direction, until agiven wafer finally returns to the load lock for unloading therefrom.

As the wafer is transported from work station to work station as abovedescribed, it is important that the wafer be supported within carrierplate assembly 18 so as to avoid any risk of mechanical damage andabrasion due to being moved about, and generally so as to be protectedfrom mechanical shock, vibration, and abrasion. To this end, wafercarrier aperture 37 is of a diameter such that both a wafer and a set ofclip assemblies 20 can be accommodated within the periphery of theaperture, and recessed and parallel with the carrier plate, therebyprotecting the wafer. The set of thin edgewise-acting clip assembliesalso is important to the formation of the thin load lock arrangement 12,and edgewise resiliently supports the wafer in an upright positionwithin plate assembly 18. An especially advantageous form of such anedgewise acting clip assembly is shown in cross section in FIGS. 4through 8, and is disclosed in detail in the aforementioned commonlyassigned copending application "Wafer Support Assembly." A set of fourclip assemblies 20 is mounted within retaining rings 41 which areremovably attached to the disc-like circular wafer carrier plate 42concentrically with each of plate apertures 37, thus forming thecomplete wafer carrier plate assembly 18. This arrangement mounts a setof clip assemblies 20 in spaced relationship within the periphery ofeach circular aperture 38. Retaining rings 41 are of U-shaped crosssection, with each having flanges 46 and 47 defining the inner and outerperipheries thereof, and clip assemblies 20 are recessed within theseflanges. Although it is preferred that four clip assemblies be usedwithin an aperture 37, it is possible to use three, or a number greaterthan four. However, a set of four has been found to provide greaterrelability than three.

As may be seen in any of the FIGS. 3 through 8, clip assemblies 20 eachinclude a block 50 of generally rectangular cross section, which may beof insulating material for applications such as sputter etch for whichelectrical isolation of the wafer is desired, and an elongated springclip 53 firmly engaged in wraparound fashion about block 50. Each clip53 includes at the end thereof opposite the block an arcuate fingerportion or tip 55, which is of a curvature in radius appropriate togripping an edge of a wafer. Extending from block 50 is proximal flatportion 56, which lies within a plane closely adjacent and parallel withthe plane defined by plate aperture 37. On the other hand, distalportion 57 is angled away from portion 56 toward the plane of plateaperture 37, and defines an obtuse angle with portion 56. This cliparrangement results in a plurality of arcuate tips 55 lying on acircular pattern of diameter somewhat less than that of a typical wafer15 (such circular pattern also lies within the plane of wafer carrierplate 42).

Wafer insertion into load lock 12 may be effected manually by simplypushing a wafer by its edge or rear face into clip assemblies 20. Thiswill, however, involve some edge rubbing of the wafer against distalportion 56, to spread apart the clips somewhat to accept the waferwithin tips 55. In order to insert a wafer without such rubbing contacttherewith, the clips must first by slightly spread, and then allowed torebound against the edge of the wafer upon insertion thereof into theload lock. Although both wafer insertion and clip spreading may be donemanually, it is far preferable to avoid all such manual operations, andthe consequent added risk of damage, error, and contamination associatedtherewith. Chamber door assembly 22 carries thereon a vacuum chuck 60centrally axially therethrough, and a plurality of clip actuating means62 near the periphery thereof. These elements, along with wafer cassetteload/unload assembly 24, provide an automated wafer loading andunloading arrangement for load lock 12 which avoids all such manualhandling of the wafers, and automates the loading process.

As thus seen in FIGS. 1 and 3, chamber door assembly 22 is attached tofront wall 32 of chamber 10 by a heavy-duty hinge 63 having a verticalaxis, to allow the door to open and close in a conventional manner to afully open position as shown, wherein the door and its inside face 64are vertical and perpendicular to the plane of chamber entrance 23, aswell as to plate assembly 18. Vacuum chuck 60, which extends axially andcentrally through the door so that the active end thereof forms part ofthe inside face 64 of the door, engages a wafer presented vertically tothe inside face of the door and holds the wafer by vacuum suction as thedoor is closed, whereupon the vacuum chuck axially extends from theinner door face as shown in FIG. 4 to carry the wafer into engagementwith clip assemblies 20. The vacuum chuck will then withdraw, and wafer15 is held in the chanmber by the clip assemblies and undergoesprocessing, and movement to the various work stations in turn byrotation of plate assembly 18. In this preferred embodiment, thevertical presentation of the wafer to the inside face 64 of the door iseffected by load/unload assembly 24, as will be further detailed below.

It should be noted that the load lock arrangement, wafer carrier plateassembly 18, and door assembly 22 need not be limited to a verticalorientation, although this is preferred to help obviate any possibilityof debris settling upon a surface of the wafer. The clip assembly,carrier plate and load lock arrangement of the invention, as well as allof the work stations, function equally well if oriented horizontally.Indeed, although the load/unload assembly 24 for the vertically-orientedwafer cassettes is meant for vertical operation, the door assembly 22could easily be made to load wafers into the load lock in a horizontalplane, yet accept wafers in a vertical orientation, by suitablemodification of its manner of mounting to the chamber wall in aconventional manner.

As noted above, it is preferable to avoid simply loading a wafer intothe clip assemblies 20 within the load lock by pushing a wafer againstthe angled distal portion 57 of the clips. In order to insert a waferwithout such rubbing contact, the clips must first be slightly spread,and then allowed to rebound against the edge of the wafer upon insertionthereof into the load lock. This is accomplished automatically as thewafer is being inserted by vacuum chuck 60 by four clip actuating means62 mounted within the door as aforementioned. Each clip actuating means62 is mounted so as to be in registration with a corresponding one ofclip assemblies 20 when the door is in its closed position. Each clipactuating means 62, shown in detail in FIG. 4, includes a pneumaticcylinder 65, a contact pin 66 which moves axially inwardly andoutwardly. Pins 66 are each in registration with one of flat proximalclip portions 56 when the door is in its closed position. With door 22closed, pins 66 are extended just prior to insertion of a wafer; or as awafer is to be withdrawn. The pressure of a pin 66 against the facingflat clip proximal portion 56 presses the clip and causes the tip 55 toswing back and outwardly, thereby releasing the clips, to facilitateinsertion or removal of a wafer without rubbing contact therewith.

During wafer unloading after completion of the wafer processing, theseoperations are reversed, with the chuck again extending and applyingvacuum to the backside of the wafer to engage same, with the clipactuating means again cooperating to release the clips, whereupon thedoor opens and the chuck retains the wafer on the inside face of thedoor by vacuum suction until the wafer is off-loaded by load/unloadassembly 24.

As we have seen, when in its fully opened position, door assembly 22 ispoised to accept a wafer for insertion into the load lock arrangement12, or it has just opened and carried a finished wafer from load lock12, which must then be unloaded from the vacuum chuck. The function ofpresenting a wafer to the door assembly 22 for loading, or for removinga processed wafer therefrom for unloading, is performed by cassetteload/unload assembly 24, which includes wafer elevator assembly 68 andwafer cassette conveyor assembly 69. Extending below and on either sideof chamber entrance 23 and attached to wall 32 of the chamber is theconveyor assembly, which moves cassettes 70 of wafers generally alongfrom the right of the entrance as shown in FIG. 1 to left. Thecooperating wafer elevator assembly 68 lifts wafers individually fromthe cassettes conveyed by conveyor assembly 69 to the operative end ofvacuum chuck 60 within the inside face 64 of door assembly 22, or lowerssuch wafers from the door upon completion of processing.

Conveyor assembly 69 includes a spaced pair of parallel rails 72 and 73extending horizontally and longitudinally across the front of waferprocessing chamber 10. The rails support and convey cassettes 70, andthe spacing of rails 72 and 73 is such that the sidewalls of thecassettes straddle the rail and enable the cassettes to be slidablymoved along the rails across the conveyor assembly. Motive power for themovement of the cassettes is provided by chain drive means 75 whichincludes various guides and gear arrangements causing a roller chain tobe moved alongside rail 72. The chain is provided at regular intervalswith guide pins 76, which engage a matching cutout on the bottom ofcassette wall 77 adjacent rail 72. Thus, the cassette is caused to moveat the same rate as the chain toward and away from elevator assembly 68,as required. A stepper motor means 80 is provided as the driving powerfor the chain means 75, to provide precise control over the movement ofthe cassettes, so that any chosen individual wafer within a cassette maybe positioned for interaction with the wafer elevator assembly 68. Aconventional memory means is coupled to stepper motor 80 and waferelevator assembly 68, which stores the location of an individual waferwithin a cassette. Thus, although several further wafers may have beenloaded into processing chamber 10 and the cassette accordingly advancedseveral positions since a first wafer was loaded, yet upon emergence ofthe completed first wafer, the stepper motor may be reversed therequired number of steps to return the completed wafer to its originalposition, then again resume its advanced position to continue itsloading function.

The cassettes 70 hold a plurality of the wafers 15 in spaced, facing,aligned and parallel relationship, and are open at the top as well asover a substantial portion of their bottom to permit access from belowand above the wafers. They must be loaded so that the front faces of thewafers, which contain the grooves, steps, and other features definingthe microcircuit components, face away from the inside face 64 of theopen door 22, and so that the rear faces of the wafers face toward thedoor assembly. This ensures that when the vacuum chuck 60 engages thewafer, no contact is made with the front face containing the delicatemicrocircuits, and that the wafer is properly positioned upon insertioninto the load lock 12 so that it will be oriented properly with respectto processing equipment within the processing chamber 10.

The wafer elevator assembly 68 is positioned below and just to the leftside of chamber entrance 23 and includes an upper guide plate 82, ablade-like elevator member 83, and an actuating cylinder 84 connectingto the lower end of member 83. Elevator blade member 83 is guided formovement up and down in a vertical path intersecting at right anglesconveyor 69 between rails 72 and 73 to inside face 64 of door 22. Guideslot 85 in guide plate 82 just below the inside face of the door in theopen position provides the uppermost guide for blade 83, while avertical guide member 86 extending below the conveyor toward theactuating cylinder also aids in retaining blade 83 on its vertical path.The width of blade 83 is less than that of the spacing between rails 72and 73, and also less than the spacing between the main walls of thecassettes 70 which straddle rails 72 and 73. Blade 83 is also thinnerthan the spacing between adjacent wafers retained in cassettes 70.

Blade member 83 is further provided with an arcuate upper end 87 shapedto match the curvature of the wafers, and a groove within this endadapted to match the thickness of a wafer and retain a wafer edgewisetherewithin. Thus, elevator blade member 83 passes between guide rails72 and 73 and intersects conveyor and cassette at right angles thereto,upon stepper motor means 80 and chain drive 75 bringing a cassette andwafer into registration over the path of the blade. As may be seen, thecassettes are constructed to allow access from below to the wafers, andto allow elevator blade 83 to pass completely therethrough. Accordinglyupon stepper motor means 80 and chain means 75 placing a cassette andwafer in registration over the path of the blade, blade 83 movesupwardly between the conveyor rails to engage from below a wafer withinthe groove of its upper end 87, and elevate the wafer upwardly to aposition in registration concentrically with and immediately adjacentinside face 64 of chamber door 22 in its open position. Note that sincethe wafers are vertically oriented, gravity aids in holding the wafersfirmly yet gently and securely in the grooved end 87 of the blade.Contact with the delicate front face of the wafer, upon which thedelicate microcircuits are defined, is therefore virtually completelyavoided, unlike the case of typical automated handling when the wafer isin a horizontal orientation. Thus the risk of damage or abrasion to thewafer is greatly lessened.

Upon arrival of the wafer at the door 22, vacuum chuck 60 engages wafer15 at its rear face by suction, and elevator blade 83 then is loweredthrough guide slot 85 and the cassette to a point below conveyor 69.Door 22 then closes with the wafer retained by the chuck 60, and thewafer is thereby loaded into the load lock arrangement 12 and chamberentrance 23 sealed simultaneously as described above for processingwithin chamber 10. Prior to completion of processing for wafer 15, stillfurther wafers may be loaded within the remaining ones of apertures 37of plate assembly 18; therefore the stepper motor and chain drive stepthe cassette one wafer position to move the next wafer serially inposition over blade 83. Blade 83 then rises to repeat its operation ofmoving this next wafer upwardly to the open door, whose vacuum chuckthen again engages that wafer for insertion into the load lock.Meanwhile, upon completion of processing for original wafer 15 byrotation in turn to each station, it is again at load lock 12, andvacuum chuck 60 again extends to the backside of the wafer while thedoor is still in its closed position, and clip actuating means 62simultaneously depress the clips to disengage same from the wafer toenable the removal thereof by chuck 60, whereupon the door is opened andthe wafer again positioned over the path of blade 83. Meanwhile, steppermotor means 80 and chain means 75 move the cassette back so that theoriginal position of wafer 15 is presented over the blade path. Blade 83then rises through conveyor rails 72 and 73 and slot 85 upwardly toengage the lower edge of wafer 15, whereupon chuck 60 releases thewafer, and enables blade 83 to lower the wafer back into its originalposition within the cassette. The cassette is then propelled forward tothe position of the next wafer to be processed serially.

Prior to the elevation of the individual wafers by the elevator assembly69 and loading into the load lock, it is desirable to insure a standardorientation for the wafers, so that the usual guide flat 91 across acord of each wafer is aligned to be lowermost in the cassettes. In thismanner, each of the wafers is assured of assuming the same position withrespect to the processing equipment within the chamber. Further, makingcertain that the guide flat is in a given predetermined position assuresthat clip assemblies 20 within plate assembly 18 will function properly,and not accidently engage a flat of the wafer instead of a portion ofthe main circular edge. To ensure such standard orientation, a pair ofopposed rollers 90 is provided which are longitudinally extended alongand between rails 72 and 73 so that the roller axes are parallel withthe rails. The rails are positioned in the path of the cassettes justprior to the position of the elevator assembly 68, so that orientationof the wafers is completed prior to their reaching the elevatorassembly. Upon passage of the cassette over the rollers, the rollers areelevated and then are driven serially and in opposed senses, oneclockwise and the other counterclockwise, and lightly contact thecircular edge of the wafers. Contact with moving rollers 90 then has theeffect of rotating the wafers within the cassettes until the guide flat91 of each wafer is positioned at a tangent to the moving rollers,whereupon contact with the roller is lost and the wafers are allpositioned with guide flats facing downwardly and in alignment,whereupon the rollers 90 are retracted downwardly.

As aforementioned, pressure plate 16 is driven against carrier plate 18and wall 32 whenever door 22 is in its opened position, to protect theevacuated interior environment of the chamber from the atmosphere. Wehave seen that FIGS. 4 and 5 show in more detail the relativepositioning of the pressure plate and wafer carrier plate, with FIG. 4showing the aforementioned sandwich arrangement of the elements definingthe load lock arrangement 12, and FIG. 5 showing the relativepositioning of the elements when the pressure plate is in its withdrawnposition. Note also that FIG. 4 shows vacuum chuck 60 in its extendedposition as the wafer is inserted into clip assemblies 20 with pins 67of clip actuating means 62 partially extended after having spread theclips; while in FIG. 5, the vacuum chuck has withdrawn, as have the pinsof the clip actuating means, and the wafer is now securely mounted inwafer carrier plate assembly 18. With pressure plate 16 withdrawn, thewafer is now ready to be rotated to subsequent processing stations. InFIG. 6, the vacuum chuck is also in the withdrawn position; however, thevacuum suction is operative, and the wafer is shown in its positionagainst the inner face 64 of chamber door 22. This is, of course, theposition of the elements of the load lock and the wafer just after thewafer has been withdrawn from clip assemblies 20, prior to its beingremoved from the load lock; or, it also represents the position of theelements just after the door has been closed and the vacuum chuck hasnot yet advanced the wafer to its position within aperture 51 of thewafer carrier assembly. Pins 67 of the clip actuating means 62 are shownbearing upon the clips just prior to depressing same to spread the clipsin order to accept the wafer therewithin.

Upon completion of loading of the load lock with a wafer 15, the loadlock is rough-pumped during a cycle lasting much less than a minute downto a level which, though still a good degree less evacuated than thechamber, does not appreciably disturb the chamber environment when thepressure plate is withdrawn as shown in FIG. 5, and the wafer 15 rotatedto the next work station. This may be effectively done in such a shorttime frame not only because the load lock is of such small volumecompared to the chamber (being only essentially that required to containthe wafer itself), but also because the outgassing load which wasintroduced therein is essentially only that of the wafer surfacesthemselves, since no ancillary support equipment is utilized fromsources outside of the load lock region, and since in any event the areaof the clip assemblies supporting the wafer within the chamber is smallrelative to the wafer. This should be contrasted with the situation ofprior art systems in which platens and other outside supports areintroduced into the load lock, which supports have considerable areawhich contributes very greatly to the gas pumpdown load. Of course, thelack of such supports introduced from the outside also contributessignificantly to a lessened risk of contamination. It should also benoted that the situation gets even better as the wafer advances to thesubsequent work stations, since that portion of the pressure plate atthe load lock region which is exposed to atmosphere (or the loadingenvironment, which preferably is enclosed in a dry nitrogen environment)does not rotate with the wafer, but rather remains in the same loadingstation location, away from the remaining work stations and moreover issealed off from the chamber environment during deposition.

While a wafer is being loaded into and/or unloaded from load lockstation 12, pressure plate 16 is in its active advanced position of FIG.4, whereby plate assembly 18 is forced against front wall 32 of thechamber, the pressure plate is similarly urging wafers in the remainingstations into contact or closer, working cooperation with the processingdevices at those stations. For example, at wafer heating station 28, thenext station beyond load lock station 12 wafer heating means 92 forpromoting wafer outgassing is provided. The wafer heating means 92,which is shown in FIG. 7, comprises a cylindrical supporting member 93of somewhat lesser diameter than the wafers and includes as the heatingelement 94 a ceramic disc in which resistance wire is imbedded in amanner to allow the surface of the ceramic disc to be heatedcontrollably and to a generally uniform temperature over its planarsurface. The wafer heating means 92 is mounted upon front wall 32 of theprocessing chamber in a sealed aperture thereof, such that the heatedsurface of the element protrudes from the plane of the chamber frontwall 32 to a slight degree. When pressure plate 16 is in its relaxedstate, the location of the pressure plate with respect to the front wallof the chamber is spaced sufficiently so that the heated surface is notvery close to the carrier plate or any wafer therewithin. However, whenpressure plate 16 is in its active forward position, the wafer carrierplate 42 is compressed against front wall 32 of the chamber so that thespacing between the heated surface and any wafer in registration withthe heating station is very close, but not so close so as to contact theheated surface, as may be seen from FIG. 7.

In a vacuum environment, the principal mechanism of heat transfer is byradiation. P-doped silicon wafers, which are widely used insemiconductor device fabrication, are quite transparent to infraredradiation. As a result, the rates of wafer temperature rise are too lowto be effective in promoting increased wafer outgassing rates during theshort outgassing cycle required in the apparatus of this invention.Because the wafers are stationary while at the wafer heating station 28,it is convenient to increase the rate of transfer of heat from theheating element 94 to wafer 15 by utilizing gas conduction heattransfer. This is accomplished by introducing through the central pipe114 as shown in FIG. 7 some fraction of the argon gas employed foroperation of the sputter deposition source directly into the spacebetween heating element 94 and wafer 15. Heat transfer is effected as aresult of argon atoms striking hot and cold surfaces alternately. Toachieve desirably high rates of heat transfer, it is necessary tointroduce the argon into heating station 28 at pressures which are inthe range approximately 100 to 1000 microns, which are one to two ordersof magnitude greater than the normal argon pressure in the main chamberof about 10 microns.

Wafer heating member 92 comprises also a backing plate 98 to whichcylindrical support member 93 is attached. A vacuum seal between backingplate 98 and chamber wall 32 is provided by O-ring 115. To avoiddegradation of the vacuum sealing properties of O-ring 115 byoverheating as a result of heat generated in heating element 94,conduits 96 and 97 passing into and through backing plate 98 areprovided to allow coolant to be flowed into and out of the backingplate, thereby cooling backing plate 98 to maintain the vacuum integrityof O-ring seal 115.

In certain applications, it will be desirable to heat and clean thewafers at the heating station by means of rf (radio frequency) sputteretch using methods which are well known to those skilled in the art. Inorder to perform the rf sputter etch operation in the short cycle timerequired in the apparatus of this invention, the required application ofrf power may cause the wafer temperature to rise to an undesirable orunacceptable level. This problem may be alleviated through the use ofgas conduction heat transfer once again, this time to transfer heat fromthe wafer to a cooled heat sink.

A suitable wafer cooling means 118, which is shown in FIG. 8, comprisesa cylindrical heat sink member 119 mounted on a backing plate 120. Avacuum seal between backing plate 120 and chamber wall 32 is provided byO-ring seal 121. To maintain the temperature of heat sink 119 at asuitably low value, conduits 128 and 129 passing through backing plate120 and into and through heat sink 119 are provided to allow coolant tobe flowed into and out of heat sink 119, thereby maintaining itstemperature at the desired level. Heat sink member 119 has a planarsurface 125 closely spaced to but not in contact with wafer 15 whenpressure plate 16 is in its active forward position. A central pipe 126as shown in FIG. 8 is provided to allow some fraction of the argon gasemployed for operation of the sputter deposition source to be introduceddirectly into the space between heat sink 119 and wafer 15. Suchintroduction of argon gas increases the rate of cooling by increasingthe rate of heat transfer from wafer 15 to heat sink 119, in the samemanner that the rate of heat transfer was increased from heating element94 to wafer 15 in the case of heating station 28, as described earlierin connection with FIG. 7.

The next station to which the wafer is advanced is coating station 14,which is mounted on the back plate 99 of the chamber, and for which anaperture 100 of circular form is made within the pressure plate, toenable the sputtering source to direct coating therethrough to a waferadvanced by carrier plate assembly 18 into registration at the coatingstation. A shutter 102 is also provided to enable the coating materialto be blocked during rotation of the carrier plate assembly and when awafer is not present at the coating station. FIG. 9 shows therelationship of the elements at the coating station 14 in more detail.It might be noted that the geometry of FIG. 9 depicts the elements intheir configuration just prior to movement by the pressure plate to itsactive forward position, to compress the wafer carrier plate againstfront wall 32 of the chamber. Thus the position of the wafer duringcoating will be closer to the front wall than illustrated in the restposition of FIG. 9, such that wafer 15 will be held in stable andstationary fixed concentric relationship with sputter source 100.

A major advantage of the manner of supporting the wafer edgewise andindividually coating same is now clear. It is well known that thesputtering process whereby a metallization coating is deposited onto thefront side of the wafer will cause the wafer to further be heated, andthus increase outgassing of the wafer at the worst possible time.However, the close coupling between the source 100 and wafer 15 with theconsequently rapid coating deposition rate (approximately 10,000angstroms per minute); the low level of contaminants (due, for example,to the lack of external wafer supports and the lack of any adjacentwafers adding outgassing products to the environment immediately aheadof the front face of the wafer); and the fact that the back-sideoutgassing products will most likely impinge on the shield structuressurrounding the sputtering source; all contribute to a much lowerconcentration of contaminants due to outgassing ending up on the frontsurface of the wafer, compared to prior configurations. In the case ofearlier batch and other load lock systems, a plurality of wafers areadjacent each other, will likely be supported upon a platen, and willnot have the benefit of a source-to-wafer geometry to allow forindividual shields upon which contamination products and coatingmaterial could preferentially combine rather than on the wafer surfaceitself, as in the present configuration.

Still other advantages grow out of the fact that the metallization ofthe individual wafer is carried out with the wafer in a verticalorientation, and further, that the metallization is done while the waferis stationary. It is clear that if any debris or particulate matter ispresent in the system, the chances of such matter coming to rest on avertical wafer surface are much reduced as compared to a case whereinthe wafer is horizontally oriented. The fact that all motion within thechamber ceases during metallization means that no mechanical motion,shock, or vibration is present which would tend to promote debrisgeneration, as, for example, by dislodgment of stray metallizationmaterial from wafer support structures, shields and other such surfaces.Additionally, in the present system, the stress on any such straycoating build-up on shields, and other internal chamber structure isfurther alleviated by lack of repeated exposure to air, a lessening ofmechanical strains due to the necessity for stopping movement during theprocessing period, and fewer moving parts. The very small structure ofclip assemblies 20 which support the wafer is not even itself exposed toair during normal operations, since the load lock is normally operatedin a dry nitrogen environment, rather than moisture-bearing air.

The close-coupled wafer-source relationship, and its stationary aspect,have additional favorable/implications for the desirable characteristicsand homogeneity of the films deposited. The local rate of deposition ata point on the wafer surface depends on the radial position and surfacetopography, i.e., whether the surface at that position is planar, or thesidewall or bottom of a step or groove, or the inward or outwardorientation of the sidewall, as will be discussed further below. Becausethe wafer is stationary with respect to the source, deposition at eachpoint proceeds at a constant rate, rather than at a time-varying ratethroughout the deposition (assuming constant power applied to thedeposition source.) Thus, deposition thickness at various points andtopographies will be radially symmetric about the axis common to theconcentrically located deposition source and wafer.

Further, as implied above, contamination levels incorporated into thecoating depend on the relative arrival rates at the wafer surface ofmolecules of contaminant background gases (such as oxygen) and atoms ofthe sputtered coating material (such as aluminum). If the partialpressures of contaminant background gases remain constant duringconstant-rate deposition, then the local contaminant level incorporatedinto the coating will be homogenous throughout the thickness of thedeposited film

By contrast, this situation has not obtained in prior art load locksystems in which wafer motion relative to the source leads to depositionrates which vary with time during the deposition period. This then leadsto inhomogeneities in contaminant level incorporated into the coatingduring film growth, which in turn has adversely affected the yields ofgood semiconductor devices from the wafer. In the case of prior artsystems in which wafers make multiple passes past the deposition source,the metal film is deposited in layered fashion, which in turn leads toan undesirable layered contaminant level profile.

Reverting now to a more detailed consideration of sputtering source 100of FIG. 9, we see that the emitting end thereof includes a ring-shapedtarget 112, which is shown schematially in dotted representation in FIG.9, but which is shown in greater detail in cross-section in theschematisized cross-section of FIG. 10. An example of such a sputteringsource may be found described in more detail in U.S. Pat. No. 4,100,055,issued July 11, 1978 to R. M. Rainey for "Target Profile for SputteringAppparatus". Such a sputtering source is also commercially availablefrom and manufactured by Varian Associates, Inc. under the registeredtrademark "S-Gun". Such sputter coating sources employ a magneticallyconfined gas discharge and require a subatmospheric inert gasenvironment as of argon. Other sputtering sources with ring-shapedtargets may also be used, as, for example, a planar magnetron source.

Positive ions from the gas discharge strike S-Gun target 112, which ismade of the source material for coating which is desired to bedeposited, for example, aluminum. Thus, source material is caused to besputtered from the target outwardly from the source. The sputter coatingprocess is carried out in the subatmospheric controlled environment ofvacuum chamber 10, within which the dominant gas is normally argon,deliberately introduced at very low pressures to sustain the gasdischarge. The argon pressure required to sustain the discharge isgenerally in the range 2-20 microns, and has been found to influencecoating quality, as will be further described below. It is found thatthe argon needed for such discharge sustenance may advantageously comefrom the argon deliberately introduced at the various wafer processingstations, as described above in connection with wafer heating station 28and wafer cooling means 118.

As may be seen from FIGS. 9 and 10, source 100 may be regarded as aring-shaped source with an inner diameter and an outer diameter, andwith a profile connecting the two diameters of generally invertedconical configuration averaging about 30 degrees. It will be noted thatthe term "conical" is an approximation, since in practice the profile oftarget 112 will erode significantly over the life of the target. FIG. 10illustrates in superimposed fashion both a typical new target profile,and that same target's profile at the end of target life. Moreover, manyprofile variations are possible; see, for example, the above mentionedU.S. Pat. No. 4,100,055; further, some useful ring sources, for example,of the planar magnetron type, will not exhibit this generally conicalprofile.

Despite such erosion, a significant fraction of the material emanatingfrom target 112 is still directed inwardly toward the axis of thesource, even when near the end of target life. In addition, somematerial from the more eroded bottom sections of the target which may bedirected outwardly will actually be intercepted by the eroded sides,from which resputtering in a generally inwardly direction may occur.Thus, even with target erosion, the source 100 may be characterized asacting as a ring-shaped source, and of effectively generally invertedconical configuration. It is believed that the generally invertedconical configuration leads to a greater efficiency of utilization ofmaterial sputtered from the source than for a planar configuration. Thisis believed to be so because a larger fraction of the sputtered materialis directed generally inwardly because of the conical configuration,resulting in a larger fraction being deposited on the wafer instead ofbeing deposited uselessly on the shielding.

As may be seen from FIG. 9, the wafer 15 is held in fixed concentricstationary and parallel relationship with respect to source 100 duringcoating as described above, being supported resiliently within therelatively thin wafer carrier or transfer plate assembly 18 by clipassemblies 20. These relationships are more analytically treated in theschematic showing of FIG. 10, which serves to help define the effectivesource target-to-wafer spacing x, the effective source diameter D_(s),and the radial position r along the wafer to be coated measured from thecenter thereof. In defining these quanities, it is useful to identify aneffective plane of the source P_(s), which is the planar level such thatby the end of target life, the amounts of material eroded above andbelow this planar level are equal. It is also useful to define theeffective source diameter D_(s) as that diameter such that by the end oftarget life, the amount of material eroded outside of that diameter isequal to the amount of material eroded away inside that diameter.Accordingly, x is analytically the distance between the wafer and theplane P_(s). A typical sputtering source 100 such as is commerciallyavailable can have actual outer and inner target diameters of 5.15inches and 2.12 inches respectively, and a target height of 0.88 inches,as shown in FIG. 10. Thus, for the erosion pattern shown, the effectivesource diameter D_(s) is approximately 4.6 inches for such a commercialsource. Likewise, it can be seen that the effective plane P_(s) of thesource is located approximately 0.5 inches below the top edge of theuneroded target.

It has been found that, surprisingly, coating of the semiconductorwafers with very good planar uniformity and superior step coverage ispossible while maintaining wafer 15 stationary as shown with respect tosource 100 during deposition, and that this may be done withinrelatively short deposition times of approximately one minute, as longas certain geometrical and positional constraints are observed, and aproper inert gas environment and pressure are maintained. Thesepreconditions for obtaining very good uniformity and superior stepcoverage, as well as the degree of improvement in these factors whichmay be expected, are illustrated graphically in FIGS. 11 through 15. Itshould be understood that the term "uniformity" as utilized herein andin the Figures is the ratio of thickness at the radial position at whichthe uniformity is being considered to the thickness at the center of thewafer. Thus, as usual, uniformity is normalized to unity at the centerof the wafer. FIGS. 11 and 12 illustrate uniformity of thickness ofdeposition on the main uppermost planar surfaces of wafer 15 as afunction of radial position r in inches, with uniformity being arelative measure, normalized as aforementioned. In FIG. 11, four curvesare shown, each being associated respectively with source-to-waferdistances x of 2, 3, 4 and 5 inches. The argon environment of thechamber is at a pressure of 3 microns. In FIG. 12, both uniformitycurves are for source-to-wafer distance x equal 4 inches; however, oneis for an argon environment of 2 microns pressure, while the other isfor an argon environment of 10 microns pressure.

FIG. 11 illustrates the surprising result that uniformity of depositionover the planar surface of wafer 15 is very good, even with no relativemotion between source and wafer, as long as the effective sourcediameter D_(s) is greater than the diameter D_(w) of the wafer. Morespecifically, as shown by FIG. 11, the uniformity of planar coverage isbetter than ±15% as long as: (a) x is approximately within the range 0.4D_(s) to 1.1 D_(s) (x=2-5" for D_(s) =4.6"); and (b) the maximum waferdiameter D_(wmax) is less than approximately 0.9 D_(s), (or a value ofr, equal to 1/2 the wafer diameter, of approximately 2.1" for a sourceof D_(s) =4.6" effective diameter).

Even better tolerances are exhibited over certain ranges of waferdiameters within the above outside limits. For example, over the rangeof ratios of wafer diameter-to-source diameter up to about 0.65 (or atradial positions r up to about 1.5"), uniformity is better than ±8%. Inother words, over a wafer diameter of 3.0", and with a source effectivediameter of 4.6", and assuming 3 micron argon pressure environment, theplanar uniformity is better than ±8%, regardless of whichsource-to-wafer spacing x is chosen within the range 0.4 D_(s) to 1.1D_(s). By restricting the source-to-wafer spacing x to the rangeapproximately 0.4 D_(s) to 0.9 D_(s) (x=2-4" for D_(s) =4.6"), theplanar uniformity is further improved to better than ±5%, as may be seenfrom FIG. 11.

The foregoing planar uniformity figures will be influenced by the argonpressure of the environment within which the deposition is carried out,but the above surprising uniformity results hold nonetheless.Considering the influence of the pressure factor more particularly, FIG.12 shows what occurs for the exemplary case of source-to-wafer distanceof x=4" and a 5" outer-diameter source (for which the effective sourcediameter D_(s) =4.6") under two conditions: a 2-micron pressure argonenvironment, and a 10-micron pressure argon environment. We see that atan argon pressure of 2 microns, a uniformity of ±10% is obtained out toa maximum wafer diameter of approximately 4.3" (radial position r ofapproximately 2.2"). Upon raising the argon pressure to 10 microns, themaximum diameter to maintain the same uniformity of ±10% is reduced byabout 16% to approximately 3.6" (radial position of approximately 1.8").Alternatively, it can be seen that for a wafer of 3.0" diameter, theuniformity is about ±4% for an argon pressure of 2 microns, and becomesapproximately ±7% for an argon pressure of 10 microns, both of which aresuperior results for many semiconductor wafer applications.

FIG. 15 is a further showing of the influence of the pressure of theargon environment. In this Figure, for a source-to-wafer spacing of 4",uniformity of planar coverage is shown as a function of microns of argonpressure for two radial positions, one at 1.5", and the other at 2.0".As expected, the innermost radial position will exhibit the highestuniformity, but both vary in similar fashion over the range of argonpressures. It will be seen that from 0 to 5 microns, the change is mostrapid at both radial positions. However, from about 5 to 15 microns,uniformity changes very little, on the order of a few percent in bothcases. Thus, it may be seen that the planar uniformity is not verysensitive to changes in argon pressure within the range 5 to 15 microns,a surprising fact which becomes important in optimizing step coverage,which is much more effected by changes in argon pressure, as will beexplained in more detail below.

In order to obtain a fully satisfactory coating, it is essential thatthe sidewalls of such features as grooves and steps within the mainplanar surface of wafer 15 be adequately coated, i.e., that good "stepcoverage" be provided. Sidewalls may be defined as those surfacesgenerally perpendicular to the main planar surface of the wafer. Stepcoverage is difficult both to specify and to measure. Scanning electronmicroscopes are used by semiconductor device manufacturers as aprincipal tool for assessing at least subjectively the adequacy of stepcoverage achieved in particular applications.

In the prior art, extensive experience taught the need for relativemotion of the wafers with respect to the deposition source to obtainadequate unformity and step coverage, as we have seen. A recent articleby I. A. Blech, D. B. Fraser, and S. E. Haszko, "Optimization of A1 stepcoverage through computer simulation and electron microscopy", J. Vac.Sci. Technol. 15, 13-19 (January-February 1978), reports excellentagreement between computer simulations of metal film deposition andscanning electron microscope (SEM) photographs of actual film stepcoverage obtained with an electron beam evaporator source plus planetaryfixture configuration, with deposition onto unheated substrates.Although the source employed in the references is a small-area thermalevaporation source, while in the apparatus of the present invention, aring-shaped sputter source 100 is employed, and further the instantsource is stationary and close-coupled, the geometrical considerationsexamined in the reference are quite indicative. Although, the depositionon various surfaces by a source as in the present invention should bemuch better than from a centrally-located source of small area as in thereference, the considerations examined in the reference neverthelessindicate that for wafer sizes of practical interest relative to thediameter of the ring source, enough shadowing would occur to place thepossibility of adequate step coverage over much of the wafer in seriousdoubt. Surprisingly, however, in the present invention, very highquality step coverage is, in fact, achieved. Although many of theconfigurations above described in connection with FIGS. 11 and 12 whichresult in very good uniformity of planar coverage also afford good stepcoverage, certain ranges and values of source-to-wafer spacing andwafer-to-source diameter relationships have been identified whichprovide still better quality in such coverage as have certain ranges andvalues of argon pressure for the coating environment.

FIGS. 13 and 14 aid in defining these optimal parameters, and plot themeasurements of thickness of sidewall coverage as a function of radialposition r. All data at each radial position are normalized to thethickness of deposition which will be obtained on the planar surface atthat radial position. Note that the horizontal axis of each graph showsvalues extending to both sides of the vertical axis. This is because,physically, a given groove defined in the wafer may have sidewalls whichface both toward the center of the wafer and away from the center of thewafer. Not surprisingly, the outwardly-facing side generally receives asignificantly thinner deposit than the side facing in, despite bothbeing generally in the same radial position. This fact is reflected inFIGS. 13 and 14, so that the left side of the horizontal axiscorresponds to radial positions for sidewalls which face outwardly,while the right side corresponds to radial positions for sidewallsfacing inwardly. It will further be noted that the vertical graph plotssidewall coverage as a percentage of the planar surface coverage, and isnormalized as indicated above. Both graphs show a curve for 3 micronsargon pressure and 10 microns argon pressure, with FIG. 13 being for asource-to-substrate distance x of 4", while that of FIG. 14 is for asource-to-substrate distance x of 3".

As is readily apparent from the curves, a further unexpected result isrevealed in that the sidewall coverage on the more difficult to coatoutwardly-facing wall is dramatically improved upon raising the pressureof the argon atmosphere from 3 microns to the 10 micron region. This istrue for a range of source-to-wafer spacings, for example, in both thecase of x=3 and x=4. In the FIG. 14 case (x=3"), sidewall coverage isincreased, for example, from less than 4% to almost 12% at a radialposition of 2.0" (corresponding to the edge of a 4" diameter wafer), andfrom 15% to 20% at the edge of a 3" diameter wafer. From FIG. 13 (x=4"),sidewall coverage is increased from approximately 9% to approximately17% at the edge of a 3" wafer. Again, we see that the same generalsource-to-wafer distances and the same kinds of relationships betweenwafer and source which were found to give rise to good planar coveragealso result in good sidewall coverage, particularly when some care istaken to consider the beneficial effect of increased argon pressure inimproving coverage of the outwardly-inwardly facing sidewall, within arange which does not seriously degrade the uniformity of planar surfacecoverage.

Further, within these general parameters, more specific ranges are ofgreat interest for the most improved sidewall coverage. Morespecifically, for source-to-wafer distances x of 0.4 D_(s) to 0.9 D_(s)(i.e., x=2-4", assuming an effective source diameter D_(s) =4.6"), andwafer diameters within approximately 0.7 D_(s) (or up to D_(w) =3.2",assuming an effective source diameter D_(s) =4.6"), not only will planardeposition uniformity be better than ±10%, but also minimum sidewallcoverage will be at least 10% of the planar coverage, or more, as longas the argon pressure is kept in the vicinity of 10 microns. Certainranges within the foregoing are still more useful. For example, as wehave seen from FIGS. 13 and 14, at a source-to-wafer distance of x=3",sidewall coverage is at least 20% of the planar coverage on out to theedge of a 3" wafer, while at x=4", sidewall coverage is at least 17% inthe same situation.

An exemplary set of results can be tabulated from the above data forargon pressures during coating of 10 microns, and forsource-to-substrate distances x in the range 0.4 D_(s) to 0.9 D_(s), asfollows:

                  TABLE I                                                         ______________________________________                                                                Maximum diameter                                      Uniformity of Minimum   of semiconductor                                      planar surface                                                                              sidewall  wafer or other                                        coverage      coverage  substrates (D.sub.wmax)                               ______________________________________                                        Better than ± 20%                                                                         --       D.sub.wmax = 4.2" = 0.9 D.sub.s                       Better than ± 10%                                                                        Greater   D.sub.wmax = 3.2" = 0.7 D.sub.s                                     than 10%                                                        ______________________________________                                    

Similar results are obtained for 4" and 5" diameter wafers. Thus, forexample, for uniformity better than ±10%, and sidewall coverage greaterthan 10%, the minimum effective source diameters D_(s) are approximatelyequal to 5.7" for 4" diameter wafers, and 7.1" for 5" diameter wafers.It is of course possible to coat 3" and 4" diameter wafer using sourcesdesigned for 5" diameter wafers. In sustained production, however,considerations of efficient utilization of source material may argue infavor of sources whose sizes are optimized to each wafer size.

It is believed that some of the observed improvements in sidewallcoverage with increasing pressure of the argon environment arise as aresult of collisions between the sputtered metal atoms and the atoms ofargon gas located in the space between the source and the wafer.Sputtered atoms are thus "gas scattered" around corners and over edgesinto regions which are shadowed from line-of-sight deposition. SEMphotographs do indeed show that the step coverages obtained using 10microns argon pressure, for example, are significantly better than thoseobtained using an argon pressure of 3 microns.

It is further known (see for example, the paper by Blech, et al.,referenced above) that step coverage can be improved by heating thesubstrates during aluminum deposition to temperatures on the order of300° C. This beneficial result arises from the increased mobility atelevated temperatures of the aluminum atoms during film growth. With theclose-coupled source used in the present apparatus, and the stationarysource-to-wafer relationship during coating, high deposition rates areachieved, and at a rate of 10,000 angstroms per minute, for example,considerable heat is generated. For example, the heat of condensation ofaluminum plus the kinetic energy of the sputtered atoms reaching thewafer surface is approximately 0.2 watts per square centimeter at theabove deposition rate. The resulting temperature rise of a typicalsemiconductor wafer during a one-minute deposition cycle may be as muchas 200° C. Thus, such elevation of wafer temperature during thisclose-coupled and stationary-configuration deposition within the presentapparatus may help cause aluminum migration to occur to a degree whichis beneficial to step coverage. Further, additional application of heatcan improve step coverage even further. However, the possibility ofclosely controlled application of even heat to wafers to be coated inorder to take full advantages of these possibilities has not heretoforebeen feasible. In prior art systems, the wafers are in uncertain thermalcontact with the wafer support structure. Close coupling between waferand source has been the exception, rather than the rule, and depositionrates for a single wafer have not been very high. Control over wafertemperatures during processing has left much to be desired.

By contrast, in the apparatus of the present invention, wafers arehandled on an individual basis. In addition, wafers are held stationarywhile at the various processing stations and are closely coupled thereto(except for the load lock station). Further, since the wafers aresupported edgewise, both sides of the wafers are accessible forprocessing. As one consequence of these features, it has now been foundpossible to provide means such as 92 to control wafer temperaturesindividually at each processing station. In particular, temperaturecontrol in the present invention is achieved as aforementioned by thegas conduction heat transfer means discussed above in connection withwafer heating station 28 and wafer cooling means 118. These meansovercome the problems of heating or cooling the wafer in an evacuatedenvironment by introducing some fraction of the argon gas (a certainamount of which is required in any event for operation of the sputterdeposition source) in the space behind the back face of the wafer, asdescribed above. Such wafer heating or cooling means may also beemployed elsewhere, for example, at the coating station itself. It isknown that not only step coverage, but also various film properties,such as reflectivity, resistivity, and contact resistances are affectedby wafer temperatures during processing. To obtain, consistently andreproducibly, a particular set of desired film characteristics requiresthat the wafer temperatures be reproducibly controlled throughout theprocessing cycle. Thus, the apparatus of the present invention providesmeans for obtaining, consistently and reproducibly, a particular set ofdesired film characteristics including step coverage through wafertemperature control throughout the processing cycle.

Referring once again to FIG. 1, the next station to which the wafer 15is advanced is a second coating station 128. In some applications, twodifferent metals are required to be deposited sequentially onto thewafer 15, the first metal being deposited at the first coating station14, and the second metal being deposited at the second coating station128. When only a single metal is employed, the second coating station128 may be left inoperative. Alternatively, coating station 128 may beemployed to double the amount of sputtering source material available,thereby doubling the time between replacement of the ring-shaped targets112. Both coating stations may be operated simultaneously with, forexample, each station operating at one-half the normal deposition rate.Alternatively, of course, one coating station may be operated aloneuntil, for example, the end of target life is reached, whereupon theresponsibility for deposition may be transferred to the other coatingstation.

The next station to which wafer 15 is advanced is the cooling station130. If the wafer temperature is not too high when it reaches thecooling station, normal radiative heat transfer may suffice to lower thetemperature of the wafer to the point that it can be safely removed fromthe vacuum environment by the end of the cooling cycle. If adequatecooling is not achieved by radiation alone, the problem may bealleviated by the use of the wafer cooling means 118 of FIG. 8, asdiscussed previously in connection with the cooling of wafers at theheating station during rf sputter etch. Once again, the use of gasconduction heat transfer, this time at cooling station 130, may play animportant role in achieving the short cycle time required in theapparatus of this invention.

The final station to which wafer 15 is advanced is the load lock station12, from which the wafer is removed and returned by means of load/unloadassembly 24 to the same slot in the cassette 70 from which it originallycame. The entire load/unload operation was described in detail earlier.

The preferred embodiment of the apparatus of this invention includes aplurality of processing stations for heating, coating, cooling and thelike, and a wafer carrier plate assembly 18 for transporting wafers onan individual basis from station to station. There are many advantageousfeatures inherent to the single wafer concept with the wafer closecoupled and stationary with respect to the deposition source.

For certain applications, or alternative embodiment includes anapparatus in which the wafer or other substrate remains affixed to theload lock door during processing, the wafer carrier plate assembly beingeliminated. A gate valve on the high vacuum side of the load lockprovides communication between the wafer and the deposition source. Atypical operation might include, for example, the steps of: waferloading; wafer heating (or, alternatively, application of r.f. sputteretch); deposition from a sputtering source; wafer cooling; and waferunloading. Gas conduction heat transfer could be advantageously employedto accelerate heating and cooling, and to provide control of wafertemperature during deposition. Although the apparatus of this embodimentlacks some of the versatility and high production rate capability of thepreferred embodiment, it does have several appealing features,including: inherent simplicity and reliability; no wafer transportinside the vacuum system; and the wafer load at risk is at theirreducible minimum of one.

In the preferred embodiment of the apparatus of this invention, thewafer 15 is presented vertically to the inside face of chamber door 22,where it is engaged by vacuum chuck 60. Vacuum chuck 60 and clipactuating means 62 are mounted within chamber door 22. Chamber door 22is the outer door of load lock arrangement 12.

In some applications it may be desirable to separate the waferloading/unloading means from the vacuum sealing means. Accordingly, afurther embodiment is one in which the wafer loading/unloading meansretracts after loading the wafer into wafer carrier plate assembly 18,following which a separate O-ring-sealed door is brought into positionto effect the outer seal for the load lock.

What is claimed is:
 1. The method of treating a wafer-like article in avacuum chamber comprising the steps of:positioning the article and aheat exchanging structure at a station within the vacuum chamber;engaging the article and said heat exchanging structure to be inpressing contact, and maintaining a gas under pressure which issubstantially less than atmospheric pressure between the article andsaid structure to facilitate the conduction of heat between the articleand said structure by said gas, said gas being inhibited from flowingfrom between said structure and article while said article and saidstructure are in contacting relationship and said heat conduction bysaid gas is being facilitated.
 2. The method of claim 1 in which gaspressure between said article and said heat-exchanging structure isgreater than the pressure within said vacuum chamber.
 3. The method ofclaim 1 in which said article comprises a wafer having two faces, andsaid gas is maintained under said pressure substantially less thanatmospheric between substantially one entire face of said wafer and saidheat-exchanging structure.
 4. The method of claim 1 in which saidpressure is less than approximately 1000 microns.
 5. The method of claim1 in which said pressure is greater than approximately 100 microns. 6.The method of claim 1 in which said pressure is approximately 100 to1000 microns.
 7. The method of claim 3 which further includes the stepof discontinuing the pressing contact between said wafer and saidstructure upon completion of wafer treatment at said station to admitgas remaining between said wafer and said heat-exchanging structure intosaid vacuum chamber.